In the field of radio receivers, there is a continuing effort to minimize the amount of tuned circuitry used. By reducing the number of tuned circuits, larger portions of the receiver may be integrated, resulting in smaller, and often less expensive, devices. This effort has resulted in widespread interest in homodyne receivers (also known as direct-conversion receivers) and low intermediate frequency (low-IF) or near-zero intermediate frequency (near-zero IF) receivers.
A well-known and common deficiency of some prior art homodyne and low-IF receivers is susceptibility to strong interfering signals. A typical front-end circuit for a radio receiver includes a filter just after the antenna input, with a bandwidth that is often significantly larger than the signal bandwidth for a given signal of interest. As a result, the signal admitted by the antenna bandpass filter may comprise one or more unwanted signals as well as the wanted signal. The unwanted signals may generate intermodulation products, among themselves and with local oscillator leakage signals appearing at the receiver input, due to square-law and higher-order distortion terms in the receiver's radio frequency (RF) circuitry. The intermodulation products may produce corrupting interference in the complex baseband signals.
Those skilled in the art will appreciate that potentially interfering signals may appear at the receiver across a spectrum extending over the total bandwidth of the receiver's RF filter or filters. Second-order (and various higher-order) intermodulation products from these signals may thus overlap the desired downconverted signal when the intermediate frequency is less than the antenna bandwidth. In the case of a homodyne or zero-IF receiver, these interfering signals may manifest themselves as a varying DC offset, which is not easily compensated by the various means commonly employed to compensate a constant DC offset. A varying DC offset is most pronounced when interfering signals are amplitude modulated, or of a bursty type, such as with time-domain multiple access (TDMA) transmissions.
A particular source of amplitude-modulated interfering signals considered herein are the signals produced by a radio transceiver's transmitter signal. This type of self-interference is present in transceivers where signals are simultaneously transmitted and received. Because a typical duplexing filter, designed to isolate the transmitter from the receiver, has limited attenuation in its stop-band, a transmitter signal can leak through or around the duplexer even when the transmitter signal spectrum is well outside the nominal bandwidth of the receiver filter. When the transmitter signal is not a constant envelope signal, but comprises amplitude modulation components, it can cause additional interference of the type described above, resulting in, for example, varying DC offset. Of course, the use of radio frequency signals comprising amplitude modulation components is becoming increasingly prevalent in wireless standards, such as those employing OFDM protocols.
The following patents issued to one of the present inventors disclose compensation of DC offset in homodyne receivers, as well as addressing other practical deficiencies such as slope and other slow drifts: U.S. Pat. No. 5,241,702 to Dent, issued Aug. 31, 1993, entitled “DC Offset Compensation in a Radio Receiver”; U.S. Pat. No. 5,568,520 to Lindquist and Dent, issued Oct. 22, 1996, entitled “Slope, Drift and Offset Compensation in Zero-IF receivers”; U.S. Pat. No. 5,712,637, issued Jan. 27, 1998, a divisional of the above '520 patent; and U.S. Pat. No. 6,473,471, issued Oct. 29, 2002, also a divisional of the above.
Various other patents disclose compensation techniques for DC offsets, including varying DC offsets, in a homodyne receiver. These patents include several issued to Lindoff et al.: U.S. Pat. No. 6,370,205 entitled “Method and Apparatus for Performing DC-Offset Compensation in a Radio Receiver,” issued Apr. 9, 2002; U.S. Pat. No. 6,449,320 entitled “Equalization with DC Offset compensation,” issued Sep. 10, 2002; and U.S. Pat. No. 7,046,720, entitled “System and Method for DC Offset Compensation in a WCDMA Receiver,” issued May 16, 2006.
In addition, U.S. Pat. No. 5,749,051, issued to current inventor Dent on May 5, 1998 and entitled “Compensation for Second Order Intermodulation in a Homodyne Receiver,” discloses compensating varying DC offsets caused by strong signals in a homodyne receiver.
All the above mentioned patents are hereby incorporated by reference herein.
As noted above, a particular source of strong interfering signals in some wireless transceivers is the transceiver's own transmitter. For example, in CDMA-based wireless communications standards such as UMTS, also known as Wideband-CDMA (WCDMA) or 3G, as well as in the 2nd-generation CDMA systems commonly known as IS-95, all of which use simultaneous transmission and reception of signals, a transceiver's receiver and transmitter are generally connected to a shared antenna via a duplexing filter. The duplexing filter, or duplexer, typically includes a transmitter filter that suppresses receiver noise generated in the transmitter and prevents it reaching the common antenna. The duplexer's receiver filter suppresses the transmit signal and prevents it reaching the receiver where it could cause overload.
However, small duplexing filters have a limited amount of stop-band attenuation, often of the order of 40-45 dB, and thus the signal reaching the receiver radio frequency (RF) amplifier is still significant. The RF amplifier amplifies the transmitter leakage signal to an even higher level, potentially to a level that can cause distortion effects in the following mixer. As a result, conventional transceivers often include a second filter between the RF amplifier and mixer to suppress the amplified transmit leakage signal. However, as cell phones and other wireless devices are required to operate in more and more frequency bands, the proliferation of filters adds size and cost. There is therefore a need to improve a receiver's tolerance of transmit leakage to allow simplification of receiver filtering.